Magnetic tape-based systems have been widely accepted in the computer industry as a cost-effective form of data storage. In a magnetic tape drive system, a magnetic tape containing a multiplicity of laterally positioned data tracks that extend along the length of the tape is drawn across a magnetic read/write transducer, referred to as a magnetic tape head. The magnetic tape heads can record and read data along the length of the magnetic tape surface as relative movement occurs between the heads and the tape.
When a tape is written to, the span of data just written is the span of the head elements. However, any expansion and contraction of the tape prior to reading results in an expansion or contraction of the space between data tracks and thus the data span. For instance, the width of the tape is known to change with changes in temperature (thermal expansion) and humidity (hygroscopic expansion). In addition, aging of the tape media tends to also change the tape's dimensions over time, adding to these effects for the cases of older tapes used for archiving data. Present tapes typically expand and contract by approximately 1 part in 1000, or 0.1%.
If recorded tracks of information are written in one environment and later read in another environment, the tape expansion or contraction will change the distances between the tracks/channels of the dataset of interest, in comparison with the distances between channels on the recording head that wrote the tracks.
Today's Linear Tape Open (LTO) format utilizes four data-bands across the ½-inch width of tape. At any one time, only one-fourth (¼) of the width of tape is spanning the head transducers. In current Linear Tape Open (LTO) systems, the heads include servo readers that are approximately 3 mm apart. The tape media also includes servo tracks having a spacing of about 3 mm, thereby defining data bands of about 3 mm. A 0.1% expansion over 3 mm results in about 3 micrometers of expansion for a data band. Accordingly, the data tracks themselves must be greater than the reader widths plus 3 micrometers or the readback will suffer from expansion- or contraction-induced misregistration. Reader transducer track widths must be made small enough to account for these dimensional changes in order that the readers are each sufficiently on track for a successful readback process. Accordingly, current tape formats are reaching their limits as far as increasing track density is concerned. To illustrate, consider the following example.
In current tape head products, read sensor width is chosen to be approximately ½ the track width on the tape. Assume that the tracks in a future system are 6 micrometers wide. The sensor is then 3 microns wide. If at the outer tracks, there are 3 micrometers of misregistration, then the readers over these tracks may be riding along the magnetic edges of the written data. Then the reader may come off the track due to uncompensated lateral tape excursions. Accordingly, the track widths (in this example) cannot be made smaller without increased risk of misreads due to tape wobble.